During the past two decades inductively coupled plasma optical emission spectroscopy (ICP-OES) has played an important role in elemental analysis ICP-OES possesses several distinct advantages over other atomic methods including simultaneous multi-element capability, relative freedom from chemical interferences, low detection limits, and a large linear dynamic range. In recent years the ICP has also been used as a source for multi-element atomic fluorescence spectrometry (AFS) (see A. Montaser and V.A. Fassel, Anal. Chem., 1976, 48, 1490 and D.R. Demers, Spectrochim. Acta, 1985, 40B, 105) and plasma source mass spectrometry (ICP-MS) (see R.S. Houk, V.A. Fassel, G.D. Flesch, H.J. Svec, A.L. Gray, and C.E. Talor, Anal. Chem., 1980, 52, 2283 and A.L. Gray, Spectrochim. Acta, 1985, 40B, 1525). However, to date, the ICP has not been successfully exploited as an atomizer for atomic absorption spectrometry (AAS).
The properties of the ICP as an atom reservoir for AAS have been investigated by Wendt and Fassel (R.H. Wendt and V.A. Fassel, Anal. Chem., 1966, 38, 337), Greenfield et. al. (S. Greenfield, P.B. Smith, A.E. Breeze, and N.M.D. Chilton, Anal. Chim. Acta, 1968, 41, 385), and Veillon and Margoshes (C Veillon and M. Margoshes, Spectrochim. Acta, 1968, 23B, 503). In addition, Magyar and Aeschbach [B. Magyar and F. Aeschbach, Spectrochim. Acta, 1980, 35B, 839) have studied the theoretical implications of using the ICP for AAS. They concluded that the ICP provided sensitivities a factor of ten poorer than those exhibited by a flame. The relatively low sensitivity of ICP-AAS can be attributed to several factors. A relatively high support gas flow rate is required to operate an ICP and this acts to dilute the sample atoms. The absorption volume in an ICP is not optimum for AAS, in particular, the absorption path length is relatively short and this combined with the high aerosol transport rate means that the residence time of analyte atoms in the absorption volume is short. Moreover, traditional AAS primarily makes use of atomic resonance lines but in the ICP the high temperature favours the production of ionic species.
In spite of these factors, a plasma environment does offer several distinctive features which suggest that it could offer several advantages over flames and graphite furnaces for atomic absorption measurements. The relatively high temperature promotes complete vaporization and dissociation and thus aids in the control of chemical interferences. In addition, radio frequency (RF) plasmas are relatively stable and easy to control. The atom reservoir temperature, and hence the characteristics of the absorption volume, can be controlled by controlling the input power to the plasma. Also, since a plasma can be made to operate with a variety of gases (e.g. Ar, He, N.sub.2, H.sub.2, etc.) the gas phase chemistry can be controlled separately from mechanisms causing energy production. Finally, the shape and extent of a plasma can be controlled through appropriate design of the external electrodes used to couple the RF power into the plasma.
U.S. Pat. No. 4,556,318, Barnes et al., discloses a spectroanalytical system which includes induction coupled plasma apparatus for exciting sample material to an atomic state for analysis.
The inventors have published in Spectrodumica Acta, 1988, a paper entitled An Atmospheric Pressure Capacitively Coupled Plasma Atomizer For Atomic Absorption And Emission Spectroscopy outlining a prior two compartment design of atmospheric pressure capacitively coupled plasma atomizer.
The development and characterization of an atmospheric pressure, capacitively coupled plasma (CCP) torch for atomic absorption spectrometry (AAS) has been specifically described D. C. Liang and M.W. Blades, Anal. Chem. 60, 27 (1988)]. Subsequent work has demonstrated that this device can also be used quite effectively as a source for atomic emission spectrometry (AES) [D.C. Liang and M.W. Blades, Abstracts, The Pittsburgh Conference & Exposition, paper No. 415 and 1140 (1988)]. The configuration described previously was designed for the analysis of small volumes of liquid samples of a size typically analyzed by electrothermal atomization AAS (5-50 .mu.L). However, the CCP can also be combined with other sample introduction techniques including laser ablation. The CCP developed for AAS was characterized by a long path length (20 cm) small diameter plasma sustained by capacitive coupling. The plasma could be operated at support-gas flow rates as low as 0.2 L/m and at radio frequency (rf) input powers between 30 to 600 W. Both the long path length tube geometry of the discharge and low support-gas flow rates acted to maximize analyte residence time in the plasma resulting in detection limits in the ng/L range.
By far the most important commercial spectral lamp for AAS and AFS is the hollow cathode lamp (HCL). The main advantages of the HCL are its very small spectral line-width and its high signal to background ratio. However, the absolute intensity of emission from the HCL is relatively low compared with the radiation from other plasma sources. To overcome this problem techniques such as direct current (dc) boosted-HCL, rf boosted-HCL, microwave coupled HCL, and high current pulsed HCL have been developed [Improved Hollow Cathode Lamps for Atomic Spectroscopy 1985, Ed. S. Caroli, Ellis Horwood Limited]. Additionally, the intensities of ion lines in HCLs are very weak, due to the dominant population of ground state atom in glow discharges [J.A.C. Broekaert, J. Anal. At. Spectrom. 2, 537 (1987)]. The factors contributing to the relatively low sensitivities of inductively coupled plasma (ICP)-AAS have been discussed previously by the inventors in the cited papers. One of the factors is that traditional AAS primarily makes use of atomic resonance lines; however there is a large population of ground state analyte ions in ICP's even at relatively low powers [G. Gillson and G. Horlick, Spectrochim. Acta 41B, 431 (1986)]. The development of an intense ion line spectral source has some significance in this area in that it could assist in the reduction of source induced shot noise, consequently improving the detection limits for plasma source AAS.
There has recently been much interest in the application of sputtering sources in atomic spectrometry [P. Hannaford and A. Walsh, Spectrochim. Acta 43B, 1053 (1988)] [H.J. Kim and E.H. Piepmeier, Anal. Chem. 60, 2040 (1988)] [A.E. Bernhard, Spectroscopy, 2 No. 6, 24 (1987)]. Sputtering is the ejection of material from a surface caused by bombardment with an energetic beam of particles [B. Chapman, in Transactions of the Conference and School on the Elements, Techniques and applications of sputtering, 1 (1969)]. DC sputtering in a glow discharge source allows one to analyze solid samples by atomizing the analytes directly from the solid state. This approach offers some advantages. The time-consuming sample decomposition step can be omitted and analysis can be carried out without addition of reagents and without any separation and/or concentration steps so the risks of introducing contaminants and the loss of the element to be determined are considerably reduced. As a consequence an analysis can be carried out quite rapidly. It would appear that the sputtering rate should be a direct function of gas pressure, since the higher the pressure the more ions which would be available for sputtering. However, sputtering is usually carried out at pressures between 5.times.10 and 1 torr since glow discharges extinguish or switch over to arc discharges at higher pressures and the main sampling mechanism in arcs is thermal evaporation.
Although rf sputtering is not widely used as a sample introduction method in atomic spectroscopy, it has long been recognized as an important technique in sputter etching and chemical vapour deposition [B. Chapman, in Transactions of the Conference and School on the Elements, Techniques and applications of sputtering, 1 (1969)]. Rf sputtering at low pressures first suggested by Wehner in 1955 [G.K. Wehner, Advances in Electronics and Electron Phys., 7, 239 (1955)]and demonstrated in 1962 [G.S. Anderson, W.N. Mayer and G.K. Wehner, J. Appl. Phys., 33,2991 (1962)] has become a standard method for etching materials in the semiconductor industry. Atmospheric pressure rf sputtering was previously used by the inventors to supply Fe to the CCP discharge for the purpose of making temperature measurements. More recently, Stephens [R. Stephens, J. Anal. At. Spectrom. 3, 1137 (1988)] has described an rf discharge between two metal electrodes at atmospheric pressure, operating in helium at a power of 5-30 W. The sputtering effect of the discharge was deduced by observing atomic emission from the plasma and atomic absorpotion within the plasma. Stephens pointed out that this device offered a convenient means of observing either emission or absorpotion for those elements for which sputtering was not inhibited by the presence of a stable oxide layer.